A touch panel system is a data input device that allows an operator to interact with information rendered on a display screen. For example, the operator can select one of multiple computer command options rendered at different locations on the display screen by touching the screen at one of the locations. A touch panel system employs a position measurement apparatus that generates an address signal indicative of the touched location. The address signal is delivered to a computer that determines from the address signal which one of the command options is selected. The object with which the operator touches the display screen is called a stylus and may include, for example, the operator's finger, a pen, or a pencil.
A touch panel system of the capacitive-type typically includes a faceplate that has on its outer major surface on optically transparent, electrically conductive coating of a preselected resistivity. The faceplate is positioned in front of the display screen of a display device so that an operator can touch the conductive coating at locations aligned with information rendered on the display screen.
The operator touches the conductive coating with a stylus having a nonzero, finite capacitance with reference to electrical ground. The stylus causes a change in the characteristics of an amplitude modulated position measurement signal applied to the conductive coating. The touch panel sysem distinguishes the location the stylus contacts from the other locations on the faceplate in accordance with the change in the characteristics of the position measurement signal.
The conductive coating on the faceplate causes a capacitive touch panel system to suffer from at least two disadvantages. First, the conductive coating is receptive to electromagnetic noise generated by the display system (e.g. cathode-ray tube flyback pulses) or present within the environment (e.g. 60 Hz background noise). Such noise susceptibility can render difficult the determination of a touch location. Second, the conductive coating causes the touch panel system to generate electromagnetic interference at the frequency of the position measurement signal. As a result, a capacitive touch panel system typically generates substantial amounts of electromagnetic noise.